This invention relates to a driving gear for all-wheel-drive vehicles, and more particularly to a driving gear for all-wheel-drive vehicles, comprising a transmission provided with a plurality of shafts parallel with the wheel axles of the vehicle, an interaxle differential gear having an axis parallel with the shafts in the transmission and adapted to output the driving force from the transmission in a divided manner, an interwheel differential gear provided concentrically with the interaxle differential gear and adapted to apply one output therefrom in a divided manner to the right and left wheels which are closer to the transmission, a bevel gear train interposed between a driving shaft and the interaxle differential gear, the driving shaft being provided so as to cross the axes of the two differential gears at right angles thereto and apply the other output from the interaxle differential gear to the right and left wheels which are more distant from the transmission, and a housing in which all of the mentioned parts are held.
German application laid-open Patent Publication No. 3116411 discloses a driving gear of this kind, in which a boundary surface between a transmission-holding housing and its adjacent housing extends at right angles to the shafts in the transmission. In this driving gear, a housing surrounding the interaxle differential gear, the interwheel differential gear and the gear train is fixed to the adjacent housing, which is adjacent to the transmission-holding housing via a joint surface which extends at right angles to the mentioned shafts.
However, the housing surrounding the two differential gears and gear train in this driving gear is made of two parts having one boundary surface, in order to assemble the driving gear easily. The axes of the various shafts extend in the boundary surface. Consequently, this housing has a plurality of boundary surfaces extending at right angles to each other. This makes it difficult to maintain the assembling accuracy of the driving gear. It is also very difficult to properly seal the portions of the housings which contact each other at right angles.